268 EMBRYOGENESIS IN PLANTS 



monocotyledons was one that was liable to happen quite frequently, 



or only very rarely. 



The development of an asymmetrical structure from a symmetrical 

 one, without necessarily involving the abortion of parts, is a very com- 

 mon phenomenon, e.g. the development of zygomorphic flowers. 

 Zygomorphic flowers, e.g. snapdragon, are initially symmetrical and so 

 remain in their peloric or actinomorphic variants. The transition from 

 actinomorphy to zygomorphy is due to changes in the reaction system 

 during growth. Where this takes place as the result of mutation, 

 it may be that a comparatively small genetical change is involved. 

 This may be true of the inception of the monocotyledonous condition. 

 If so, it would follow that the monocotyledons may well be a poly- 

 phyletic group. 



REDUCED AND ANOMALOUS MONOCOTYLEDON EMBRYOS 



The monocotyledons aff'ord some unique examples of greatly 

 reduced embryos, e.g. in the Orchidaceae. The mature seed in the 

 orchids is very small and contains a minute embryo which is almost 

 gemma-like in its simple construction and absence of diff'erentiation, 

 Fig. 70. In the family as a whole, however, the embryogeny shows 

 considerable variation. In some species, e.g. Epipactis paJmatus, there 

 is very little suspensor development, but in others, e.g. Orchis latifolia 

 and Goodyera, Fig. 70, a long filamentous suspensor, septate in the 

 former, undivided in the latter, is present. 



Even the most ardent disciple of embryonomic types must experience 

 difficulty in assigning the various Orchidaceous embryos to an appro- 

 priate group. Johansen (1950) has expounded these difficulties and has 

 classified orchid embryos according to the presence or absence of a 

 suspensor and of embryonal tubes, and whether these tubes arise 

 directly or indirectly from the suspensor initial cell. On this basis, 

 thirteen different developmental patterns can be distinguished. 

 Herminium monorchis has a well-developed filamentous suspensor 

 which projects well down into, and through, the micropyle. In 

 Epidendrum ciliare there is sustained growth and cell division in the 

 terminal segment, a rather curious, elongated and somewhat bulky 

 embryo being formed. Fig. 70. SobraUa macrantha eventually forms a 

 somewhat elongated, bulky embryo. Near the distal end there is an 

 indentation in one side, presumably the site of the shoot apex, the 

 terminal region being probably a primitive or potential cotyledon. 

 Vanda, Cymbidium, Phalaenopsis, Stanhopea, and other genera show 

 remarkable growth developments of the basal or other suspensor cells 

 into haustorial embryonal tubes, Fig. 70. These tubes eventually 

 wither about the time of seed dispersal. The mature embryo in Vanda 



